The invention relates generally to optical cross-switches and more particularly to a detector for monitoring signals in optical cross-switches.
In the past, telecommunications and data communications networks have traditionally relied on electrical signals transmitted electrically on conductive lines. As higher and higher data exchange rates are required, conductive lines are no longer sufficient and increasingly the data is transmitted through the use of optical signals through optical fibers. Equipment for efficiently generating and transmitting the optical signals has been designed and implemented, but the manufacturability of optical switches for use in telecommunications and data communications networks is problematic.
Fouquet et al. (U.S. Pat. No. 5,699,462), which is assigned to the assignee of the present invention, describes a switching matrix that is used for routing optical signals from any one of a number of parallel input optical fibers to any one of a number of parallel output optical fibers.
Referring now to FIG. 1 (PRIOR ART), therein is shown an isolated optical switching element 10 formed on a substrate 12. The substrate 12 is of silicon or silica. The optical switching element 10 includes planar waveguides defined by a lower cladding layer 14, a core 16, and an upper cladding layer 18. The core 16 is primarily silicon dioxide, but other materials that affect the index of refraction of the core may be used. The cladding layers 14 and 18 are formed of a material having a refractive index that is substantially different from the refractive index of the core material, so that optical signals are guided along the core material.
In the manufacturing process, the core 16 is patterned to define an input waveguide 20 and an output waveguide 26 of a first waveguide path and to define an input waveguide 24 and an output waveguide 22 of a second waveguide path. The upper cladding layer 18 is then deposited over the core 16. A trench 28 is etched through the core 16 at the intersection of the first and second waveguide paths and the two cladding layers 14 and 18 to the substrate 12. The waveguide paths intersect the trench 28 at an angle of incidence greater than the critical angle of total internal reflection (TIR) when the trench 28 is filled with a fluid, which can be a liquid, plasma, or a gas, having a refractive index which closely matches the refractive index of the waveguides; thus, light is transmitted to a xe2x80x9cthroughxe2x80x9d path to the output waveguide 26 when fluid is present in the trench 28.
Thus, TIR diverts light from the input waveguide 20 at a region 27 to a xe2x80x9creflectedxe2x80x9d path to the output waveguide 22, unless an index-matching fluid is located within the gap between the aligned waveguides 20 and 26. The trench 28 is positioned in a xe2x80x9cforwardxe2x80x9d position with respect to the four waveguides 20, 26, 24, and 22 such that one sidewall of the trench 28 passes through or is slightly offset from the intersection of the axes of the waveguide paths on the side away from the input of the input waveguide 20.
Referring now to FIG. 2 (PRIOR ART), therein is shown a plurality of the optical switching elements 10 in a 4 times 4 matrix 32. In the 4 times 4 matrix 32, any one of four input waveguides 34, 36, 38 and 40 may be optically coupled to any one of four output waveguides 42, 44, 46, and 48. The switching arrangement is referred to as xe2x80x9cnon-blocking,xe2x80x9d since any free input waveguide can be connected to any free output waveguide regardless of which connections have already been made through the switching arrangement. Each of the sixteen optical switches has a trench that causes TIR in the absence of an index-matching fluid at the gap between collinear waveguides, but collinear waveguides of a particular waveguide path are optically coupled when the gaps between the collinear waveguides are filled with the refractive index-matching fluid. Trenches in which the waveguide gaps are filled with fluid are represented by fine lines that extend at an angle through the intersections of optical waveguides in the array. On the other hand, trenches in which the index-matching fluid is absent at the gaps are represented by broad lines through a point of intersection.
For example, the input waveguide 20 of FIGS. 1 and 2 (PRIOR ART) is in optical communication with the output waveguide 22 as a result of reflection at the empty gap of the trench 28. Since all other cross points for allowing the input waveguide 34 to communicate with the output waveguide 44 are in a transmissive state, a signal that is generated at the input waveguide 34 will be received at output waveguide 44. In like manner, the input waveguide 36 is optically coupled to the first output waveguide 42, the third input waveguide 38 is optically coupled to the fourth output waveguide 48, and the fourth input waveguide 40 is coupled to the third output waveguide 46.
There are a number of available techniques for changing an optical switch of the type shown in FIG. 1 from a transmissive state to a reflective state and back to the transmissive state. One method of changing states involves forming and eliminating the gap by forming and removing vapor bubbles in a refractive index-matching fluid. A plurality of heating elements are used where the application of heat to a trench forms the vapor bubble to remove the fluid and the removal of the heat causes the vapor bubble to collapse and return the fluid. The heating elements and their control are well known in the art and are used in ink jet printers.
The refractive index-matching fluid is supplied from a reservoir under the trench and resides within the trench in the waveguide paths until a vapor bubble is formed to create an index mismatch and cause light to be reflected at the sidewall of a trench. Collapsing the vapor bubble returns the switch to the transmissive state. A bubble forms in less than 1 ms when heat is applied and collapses in less than 1 ms when heat is removed.
In the optical switching element 10, light coming in is from an independent outside source and the light going out goes to an independent outside receiver, neither of which can be tapped for information. Unlike an electrical circuit where it is possible to make a parallel test connection, in optical circuits, parallel connections cause losses in the signal. Further, an external coupler is required for each waveguide and must be attached for each monitoring operation, which makes uninterrupted monitoring of the optical switching element 10 costly and time consuming.
Those skilled in the art have long sought, with little success, a way to be able to perform inexpensive uninterrupted monitoring of optical switches.
The present invention provides a method of monitoring a light signal in an optical switch having a first waveguide and a second waveguide that intersect a first trench such that optical coupling is dependent upon a fluid being at the first trench, and having a test waveguide that intersects a second trench such that optical coupling is dependent upon the fluid being at the second trench. By changing the fluid at the first trench to switch the optical switch to cause a light signal to be reflected from the first waveguide into the second waveguide and by partially changing the fluid at the second trench to switch the optical switch to cause a portion of the light signal to be reflected from the first waveguide into the test waveguide. The portion of the light signal in the test waveguide can be monitored using a light monitor for inexpensive uninterrupted monitoring of the optical switch.
The present invention further provides an optical switch for a light signal monitoring system. The system includes a support structure having a first waveguide and a second waveguide that intersect a first trench such that optical coupling is dependent upon a fluid being at the first trench, and a test waveguide that intersects a second trench such that optical coupling is dependent upon the fluid being at the second trench. A mechanism is provided for changing the fluid at the first trench to switch the optical switch to cause a light signal to be reflected from the first waveguide into the second waveguide and a mechanism for partially changing the fluid at the second trench to switch the optical switch to cause a portion of the light signal to be reflected from the first waveguide into the test waveguide which can be monitored using a light monitor for inexpensive uninterrupted monitoring of the optical switch.
The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings.